On demand display mechanisms are uncommon and are always relatively complex.
CH Patent No. 693 155 in the name of Andreas Strehler discloses a mechanism for displaying several different physical quantities, using a coupling and differential gear. This device requires a certain thickness due to the existence of a planetary gear whose axis is perpendicular to the plate.
EP Patent No. 1 959 317 in the name of Maurice Lacroix SA proposes an on demand display device for displaying several data using a minimum number of hands. A switchable transmission mechanism includes a transmission pinion driven by a first wheel set to represent a first piece of information to be displayed. An element with a first fly-back heart is freely mounted on the transmission pinion, and driven, depending upon the particular case, either by the first wheel set or by a second wheel set to represent a second piece of information to be displayed. A second fly-back heart is secured to the transmission pinion which carries a freely mounted transmission wheel. This transmission wheel carries a first/second hammer pre-stressed by a first/second pre-stressed spring against the first/second heart.
A switching wheel, rotating on the periphery of the transmission wheel, carries a first/second cam acting on the first/second hammer so as to remove alternately the contact between the first/second hammer and the first/second heart, in order to switch the position of the transmission wheel according to the first/second piece of information to be displayed. An intermediate control wheel, freely mounted about the transmission pinion, is in mesh with the switching wheel and secured to a control wheel, controlled by a transmission mechanism control device.
In a first embodiment, the element including the first fly-back heart is a coupling disc freely mounted on the transmission pinion using a friction coupling, and driven by the first wheel set. Thus, the first heart represents the measured time, and the second heart mounted on the transmission pinion represents the current time. The coupling disc carries a reset heart via cooperation with a reset hammer. This coupling disc may be prevented from rotating by a chronograph control mechanism with a clamp for gripping or releasing each disc.
In a second embodiment, the element including the first fly-back heart is a wheel, freely mounted on the transmission pinion, and driven by the second wheel set. Thus, the first heart represents the second piece of information to be displayed, corresponding to the second wheel set, and the second heart, mounted on the transmission pinion, represents the first piece of information to be displayed, corresponding to the first wheel set.
This mechanism, applied to a chronograph, includes a first stop-start push-button, a second reset push-button, and a third switching push-button for switching the state of the transmission mechanism. This type of mechanism remains complex, and above all voluminous, and it is difficult to adapt to an existing movement, since it requires significant alterations, particular as regards the crown for the third push-button. It is difficult to adapt as an additional mechanism and it requires significant space in the thickness of the movement.
GB Patent No. 2 266 791 in the name of GEORGE DANIELS discloses an additional chronograph display mechanism, including a stop/start push-button and a reset push-button, wherein zero resetting is achieved via cooperation between a hammer and a cannon-pinion heart which is coupled or uncoupled by the stop/start push-button.
EP Patent No. 1 136 894 in the name of DUBOIS TECHNIQUE HORLOGERE discloses a fly-back mechanism with a dedicated push-button and a friction coupling. A second uncoupling mechanism uncouples the drive mechanism when the fly-back is reset, and when pressure is maintained on the push-button.
These high performance mechanisms are, however, quite complex and difficult to make and adjust.